1. Field of the Invention
This invention relates to a movable disk printer and in one of its aspects to such a printer in which printing is provided while at least a movable carriage on which the movable disk is mounted is on-the-fly. More specifically, this invention relates to the synchronization of a movable disk which rotates for a variable distance with the carriage moving at a variable velocity so that the moving carriage reaches a selected print position coincident with the rotatable disk's arrival at the print character selected for said print position.
2. Description of Prior Art
Printers which utilize a rotating disk with characters on the periphery thereof are well known. Several of such printers are commercially available. Rotating disk printers can be divided in categories by either focusing on how the disk rotates or by focusing on how the carrier traverses.
Focusing on how the disk rotates, such printers can be divided into a first category where the disk constantly rotates and into a second category where the motion of the disk is intermittent. In printers with a constantly rotating disk, printing takes place when the hammer strikes the rotating disk. Rotation of the disk is not stopped each time a character is printed. In printers with a disk that intermittently rotates, the disk is rotated to the desired print position and then stopped. There is no disk rotation while printing takes place.
An alternate division of disk printers can be made by focusing upon the motion of the carrier. In some printers, the traverse of the carrier is stopped each time printing takes place. In other printers, the carrier is moving at the instant when printing occurs. In both the type where the carrier is moving when printing occurs and in the type where the carrier is stopped when printing occurs, the disk may or may not be rotating at the time of printing. In some printers where the carrier is moving at a fixed speed when printing takes place, the carrier is slowed down and stopped between print positions in order to give the rotating disk time to move to the desired character.
The following are some of the issued and pending patents which show rotating disk printers:
The Willcox U.S. Pat. No. 3,461,235 issued Aug. 12, 1969 shows a disk printer with a constantly rotating disk. The carrier stops at each print position.
The Ponzano U.S. Pat. No. 3,707,214, issued Dec. 26, 1972, discloses a disk printer which has separate controls for a print wheel and its carrier. The print wheel and the carrier move by the shortest distance at the next selected position. The print wheel and the carrier stop at each print position.
The Robinson U.S. Pat. No. 3,356,199, issued Dec. 5, 1967, describes a rotating disk printer wherein the disk is constantly rotating. The type elements on the disk are in a particular spiral configuration. The carrier also moves at a constant speed which is synchronized with the motion of the disk in such a manner that the desired character can be printed in each print position.
The Giani U.S. Pat. No. 3,742,845, issued July 3, 1973, shows in FIG. 11 a drum printer which has a constantly rotating drum. It is suggested that this drum could be mounted on a carrier. The carrier would have to stop at each print position in order to give the rotating drum time to rotate to the desired character.
The Cahill U.S. Pat. No. 3,794,150, issued Feb. 26, 1974, discloses a drum printer which includes an incrementing carrier. The carrier stops at each print position until the drum rotates to the desired position.
Application Ser. No. 535,647 of Jensen et al, filed Dec. 23, 1974, discloses a carrier control system for a start-stop disk printer in which the carrier normally traverses at a predetermined speed. Printing always occurs at the same predetermined speed; however, if there is not sufficient time to rotate the disk to the next desired character, the carrier is slowed down between print positions and then returned to the predetermined speed.
U.S. Pat. No. 4,030,591, Martin et al, issued June 21, 1977, discloses a rotating disc printer where the carrier is moving at a variety of velocities when the printing by the firing of the print hammer takes place. Thus, the firing of the print hammer must be timed dependent on the velocity of the carrier or carriage at the particular instance.
In U.S. Pat. No. 3,858,509, issued Jan. 7, 1975, a rotating disk printing apparatus is disclosed in which the striking force applied to the hammer can be varied between "light" and "hard". However, in that patent the printing is not done on-the-fly and there is no need to coordinate the speed of the carriage and the travel time of the print hammer to insure that the position of the character to be printed is at the print impact point at the time it is caused to strike the printing medium.
U.S. Pat. No. 4,035,781, L. H. Chang, issued July 12, 1977, mentions a procedure in a printer wherein upon a failure to print, at least one retry to print is made before the apparatus is stopped for an error correction routine. This patent does not involve on-the-fly printing wherein the carrier is never stopped. In the apparatus of the patent, the carrier appears to stop at each print position. Thus, it appears to be unrelated to the problem of synchronization of time related parameters in on-the-fly printers.
Further developments with rotating disk printers covered in a copending application Kane et al, Ser. No. 863,450 filed Dec. 22, 1977, the details of which are included in description of the embodiment of the present invention, relate to rotating disk printers in which the carrier is moving at a variety of velocities, the rotatable character disc is rotating over a variety of distances and the print hammer is driven at a variety of forces in order to achieve consistent and high print quality. Thus, the approach in the copending application adds a further element, i.e., variable hammer force which unlike the apparatus of U.S. Pat. No. 3,858,509 must be coordinated with a variable carriage velocity and variable disk rotation distance in order to achieve the desired synchronization of selected printed character with the selected carrier print position. Actually, as set forth in said copending application and hereinafter in the detailed description of the present application, the variations in impact force of the hammer are manifested by corresponding variations in hammer flight time. Thus, in order to achieve synchronization for printing a character, the apparatus depends on the synchronization of three time-related variables: the variable distance the character disk must rotate, the variable velocity of the carriage and the variable flight time of the hammer.
To a great extent the purpose of on-the-fly printing is meeting the ever-increasing demand in the printer field for higher and higher printing speeds, in excess of 50 characters per second and approaching 100 characters per second. However, in order for on-the-fly printing to operate at its maximum speed potential, it is necessary that the apparatus controls coordinate at least two time related variables into synchronization. Otherwise, printing errors will of course occur. Since the time related variables such as the time it takes for the rotating disk to go from one character to another or the time it takes for the carrier to move from a first to a second print position are subject to transient variations, it becomes increasingly difficult to increase the printing speeds of the printers and still remain within the allowable tolerances of such transient variations. Customarily, this is done by operating the apparatus at a rate within the timing tolerances of the slowest time related parameter to be coordinated. In the case of the high speed printer, this is usually the time required for the rotating character disk to rotate the distance from a given character to the next selected character. Printing does not take place until a period of time is permitted to elapse sufficient for the rotating disk to have reached the next selected character. However, this approach does impose a significant limitation on potential operating speeds, particularly since the rotating disk subject to transient variations requiring relatively large tolerances for the time required to carry out a particular step.